In mobile communication systems, it is normal for a fixed station to transmit within a given frequency range and to receive within a frequency range that differs from the transmitting range. In certain applications, both transmission and reception are effected with the same antennas. The reason for this is to be found in the general desire to operate from a base station in a mobile communications tower in an area that is divided into separate sectors while utilizing diversity. The use of separate antennas for transmission and reception would result in an excessively large number of antennas.
In view of the relatively long downleads that are involved in this regard, there is an interest in amplifying the received signals already in connection with and close to the antenna, particularly in the case of fixed tower-mounted stations. In this regard, it is necessary to mutually separate the received incoming signals with the aid of filters, which must have the highest quality in order to bring losses down to a minimum. In an earlier known construction, the transmitter signal has been separated from the receiver signal with the aid of two signal paths, one for the transmitter signal and one for the receiver signal, each of which is connected to respective antenna and uplead sides through a duplex filter. Each duplex filter is comprised of two filters tuned to respective transmission and reception frequency bands. The advantage with this construction is that only one downlead to the ground mounted station is required. However, it can also be chosen to uplead the transmission power and downlead the receiver signal in a respective coaxial cable, thereby enabling a saving in ground positioned filter stages.